Cydia for 3.0 is finally here! Currently the only way to get Cydia 3.0 on your device is to jailbreak with a newly modified QuickPwn.
This new custom QuickPwn is compliments of HoZy, lilstevie, saurik, and iphoneappsru. This QuickPwn includes all bundles needed to jailbreak your iPhone 3G, iPhone, and iPod 1G.

If you upgrade to 3.0, it will change your baseband and yellowsnow unlock will be non-functional. Furthermore, as of now you will not be able to downgrade your baseband unless you are on a 5.08 bootloader and be stuck without a soft unlock.